[clang] [clang][modules] Allow module maps with textual headers to be non-affecting (PR #89441)

[Sam McCall via cfe-commits]

https://github.com/sam-mccall approved this pull request.

This makes sense to me. It corresponds to our build structure and fixes the new build failures we saw after 0cd0aa029647c8d1dba5c3d62f92325576796fa2.

Really appreciate your work on this!

https://github.com/llvm/llvm-project/pull/89441